There are still better and worse ways to frame an issue. Simply asking who is a lot of the benefit; if you speak in terms of rights and always consider whose responsibility is (proximally and ultimately) to guarantee that those rights are fulfilled, that's a lot of the benefit.

But there's a key difference between Alice and Bob's conversations even as you portrayed them. In the first, Bob rejected the entire idea of food security. In the second, Bob only rejected the idea that everyone had the responsibility to feed everyone else.

If Bob had said, "Nobody has the responsibility to give anyone else food," then Alice could quickly have pointed out that parents have that responsibility with respect to their children.

If Bob had said, "Nobody has the responsibility to give any adult food," then Alice could have questioned whether if Carl was here, and starving, Bob and Alice should not try to help him?

Bob could adamantly say, "No, let Carl starve."

Then, at least, the heartlessness of Bob's position is clear.

But Bob has to confront it head-on. "I disagree that everybody has that right." let him escape from confronting it quite so directly.

On the other hand, when Alice says something like, "Every person has a responsibility to feed every other person, for free if need be," then Bob can quite sensibly reply, "That's crazy--if the whole world stops making food and comes to me, why should I have to starve myself to reward their foolishness?"

And so Alice's position is also revealed to be problematic.

I just think the discussions end up being far more productive.